1. Field of the Invention
The present invention relates to a foreign substance inspection apparatus.
2. Description of the Related Art
In typical manufacturing processes for manufacturing devices such as semiconductor devices or liquid crystal devices, an exposure apparatus transfers a circuit pattern formed on a reticle onto a resist-coated wafer.
If a foreign substance is present on the reticle in the transfer process, the foreign substance is also transferred onto the wafer. This degrades the yield of device manufacturing.
In particular, when a plurality of shot regions on the wafer are exposed to transfer the circuit pattern by step and repeat, if the foreign substance is present on the reticle, the foreign substance is transferred in all shot regions. This considerably reduces the yield of the device manufacturing.
Hence, it is important to detect the presence of the foreign substance on the reticle in the device manufacturing processes. In many cases, a foreign substance inspection method using a property that the foreign substance isotropically scatters light is employed (see Japanese Patent Laid-Open Nos. 7-43312 and 7-5115).
For example, Japanese Patent Laid-Open No. 7-43312 discloses a technique in which parallel light is obliquely incident on a surface to be inspected of a reticle, and scattered light from a foreign substance is guided to a one-dimensional image sensor by a lens array. The surface to be inspected of the reticle is inspected by forming an image of the foreign substance on the one-dimensional image sensor by the lens array.
FIG. 9A is an illustration showing a basic structure of an optical system of a foreign substance inspection apparatus. In order to simplify the description, only an optical system for foreign substance inspection on a blank surface of a reticle is described. The foreign substance inspection apparatus, however, also has an optical system for foreign substance inspection on a pellicle film. The pellicle film protects a circuit pattern surface of the reticle from a foreign substance. The pellicle film is attached to a reticle 1 using a pellicle frame 2.
An irradiating unit 4 which irradiates the reticle 1 with irradiating light 45 includes a semiconductor laser 41, a collimator lens 42, and a λ/2 plate 43. The collimator lens 42 collimates divergent light emitted from the semiconductor laser 41 to be parallel light. Then, the λ/2 plate 43 polarizes the parallel light to be polarized light having a polarization direction parallel to a plane containing an optical axis of an irradiation optical system and an optical axis of a detection optical system.
The irradiating unit 4 emits the parallel light to be obliquely incident on a blank surface 1a (surface to be inspected) at an angle θ, which is nearly parallel to the blank surface 1a. Accordingly, a linear irradiation region 5 is formed on the blank surface 1a. 
If a foreign substance 3 is present in the irradiation region 5, the foreign substance 3 causes scattered light. An imaging lens 71 for receiving scattered light has lens elements arranged in a longitudinal direction of the irradiation region 5. The imaging lens 71 condenses the scattered light on a line sensor 72. The imaging lens 71 forms an image of the irradiation region 5 on the line sensor 72. The imaging lens 71 is constituted by a gradient index lens array. The imaging lens 71 and line sensor 72 are collectively identified as a detecting unit 7.
Referring now also to FIG. 9B, an optical unit 10 including the irradiating unit 4 and the detecting unit 7 of FIG. 9A linearly scans perpendicularly to the longitudinal direction of the irradiation region 5 in a direction along the blank surface 1a, that is, in the X direction, to perform foreign substance inspection for the entire blank surface 1a. 
Unfortunately, with the above-described foreign substance inspection apparatus, the irradiating light may enter the reticle from the surface of the blank surface 1a due to refraction. Diffracted light from the circuit pattern may enter the detecting unit 7, and the detecting unit may erroneously detect the diffracted light as the scattered light from the foreign substance.
Referring now also to FIG. 10, light paths causing erroneous detection when viewed from above the reticle and from a measurement surface (X direction) of a side surface 1c of the reticle are illustrated. The irradiating unit 4 forms the linear irradiation region 5 on the blank surface 1a. Since the incident angle to the blank surface 1a is large, a major part (90% or higher of light quantity) of light is reflected, whereas a part of light enters the reticle 1 due to refraction. When the light is refracted at a position P on the blank surface 1a, and is emitted to a line-and-space circuit pattern 102 patterned in the X direction, the circuit pattern 102 produces diffracted light 103L and 103R.
If the light is obliquely emitted to a line-and-space pattern, diffracted light advances in an arrangement direction of the pattern with reference to light specularly reflected by the pattern. Since the light incident on the reticle at an angle nearly parallel to the reticle and refracted is emitted to the pattern, if diffraction with the pattern occurs again when the light enters the reticle at the position P, the diffracted light is totally reflected by the blank surface although it reaches the blank surface.
Similarly to this, although the light totally reflected by the blank surface reaches a region of the pattern surface, if no pattern is present in the region, the light is totally reflected in the region. Also, the total reflection may be caused by a farthermost side surface 1b and a side surface 1c of the reticle 1 depending on the density of the circuit pattern 102.
As described above, the diffracted light 103L may be totally reflected by the pattern surface (of any of a light shielding film portion, a glass portion, and a semitransparent portion), the blank surface, and all the side surfaces of the reticle unless the circuit pattern is irradiated again to cause a diffraction phenomenon. Thus, the light quantity of the diffracted light is not decreased.
Referring to FIG. 10, the diffracted light 103L may return downward (Z direction) in the irradiation region 5 after the total reflection is repeated. If a line-and-space pattern 104 arranged in the Y direction is located at the position, the pattern 104 may cause diffracted light 105, and the detecting unit 7 (illustrated in FIG. 9A) may detect the diffracted light 105. The phenomenon is described below with reference to FIG. 11.
Referring now also to FIG. 11, there is shown a view when FIG. 9A is viewed from the irradiating unit 4. A dotted line plots a light path from the side surface 1c to the pattern 104 of the diffracted light 103L repeating the total reflection. Since the pattern 104 is a line-and-space pattern arranged in the Y direction, the inclination of the diffracted light 105 about the X axis is changed with reference to the specularly reflected light. Accordingly, in FIG. 11, the diffracted light 105 seems to be aligned with the specularly reflected light, however, in FIG. 10, the incident angle of the diffracted light 105 to the blank surface may be smaller than the critical angle, and thus the light may exit to the air. Also, the diffracted light 105 may have an angle close to the optical axis of the imaging lens 71 depending on the density of the circuit pattern 104. The line sensor 72 may detect the light, and erroneously detect it as scattered light from a foreign substance.
The diffracted light 103R also repeats the total reflection similarly to the diffracted light 103L. When the diffracted light 103R is reflected by the side surface 1b of the reticle and enters a circuit pattern region 101, the diffracted light 103R gradually disappears because diffracted light is produced in the circuit pattern region. The produced diffracted light would not enter the detecting unit 7, thereby not causing erroneous detection.